A. Technical Field
The present invention relates to a process for purification of exhaust gases and a catalyst used for purification of exhaust gases in this purification process. Specifically, the present invention aims at: a process for purification of exhaust gases to efficiently purify carbon monoxide (CO) as contained in exhaust gases as discharged from such as various combustion apparatuses such as boilers, gas turbines, diesel engines, and gas engines; and a catalyst usable for purification of exhaust gases in such a purification process.
B. Background Art
The combustion exhaust gases, as discharged from various combustion apparatuses such as boilers, gas turbines, diesel engines, and gas engines, generally contain harmful components such as CO, NOx, SOx, and unburned-fuel-derived volatile organic compounds, though depending on such as their respective combustion apparatuses and operational conditions. As to these combustion apparatuses, there are many cases where combustion is carried out under conditions where the quantity of air as supplied during combustion is rendered larger than the theoretical air quantity necessary for perfect combustion of fuel gases in order that the combustion efficiency or thermal efficiency may be enhanced and further that the quantities of harmful components such as CO and NOx may efficiently be decreased. The quantities of the harmful components such as CO and NOx as contained in the combustion exhaust gases can considerably be decreased by such as the above control of the combustion state, but such as CO and NOx still remain on a level recognized as harmful. Therefore, these residual harmful components need to also efficiently be purified, so it becomes necessary to develop a catalyst for purification of exhaust gases and a process for purification of exhaust gases which are able to function effectively on a higher level of purification. Specifically, these combustion exhaust gases contain a large quantity of oxygen, corresponding to the excessive air quantity, and a large quantity of water vapor resultant from combustion, so the purification of the harmful components such as CO and NOx as contained in the combustion exhaust gases needs development of a catalyst for purification of exhaust gases and a process for purification of exhaust gases which are able to function still effectively even if the combustion exhaust gases contain the above large quantity of oxygen and the above large quantity of water vapor resultant from combustion.
Hitherto for purification of combustion exhaust gases as discharged from combustion apparatuses for combustion under conditions near the theoretical air ratio to fuel, for example, a trinary catalyst such as Pt, Rh/alumina catalyst is proposed and a process for purification of exhaust gases with this catalyst is actually put to practical use. This process for purification of exhaust gases is carried out by passing the above exhaust gases through the Pt, Rh/alumina catalyst (honeycomb catalyst) under conditions of about 500 to about 700° C., whereby CO, NOx, and the unburned volatile organic compounds are removed from the exhaust gases.
However, the above process for purification of exhaust gases targets almost oxygen-free exhaust gases for purification and also has a premise of being carried out under conditions where the purification temperature is in the range of about 500 to about 700° C., and the above catalyst can sufficiently display its function in such a purification environment. Therefore, the above catalyst cannot adapt effectively to or sufficiently display its function in a process for purification of exhaust gases which process targets, for purification, exhaust gases that contain oxygen and water vapor in large quantities and are discharged at an exhaust gas temperature of about 300 to about 500° C., and which process is intended to remove the harmful components such as CO.
On the other hand, as to processes for purification of exhaust gases as intended for making such as CO in the exhaust gases harmless wherein the exhaust gases are discharged at a temperature of about 300 to about 500° C. and contain oxygen and/or water vapor in large quantities, for example, there are known such processes as disclosed in JP-A-241467/1995 and JP-A-241468/1995 for oxidation removal of CO from exhaust gases of dilute combustion gas engines. Specifically, JP-A-241467/1995 discloses a catalyst and a process for making the CO in the exhaust gases of the dilute combustion gas engines harmless by oxidation, and these are characterized in that the catalyst as used is a Pt/alumina catalyst as supported on a honeycomb support wherein the quantity of the supported Pt is in the range of 1.2 to 2.5 g/liter. On the other hand, JP-A-241468/1995 discloses a catalyst and a process for making the CO in the exhaust gases of the dilute combustion gas engines harmless by oxidation, and these are characterized in that the catalyst as used is a Pt, Pd/alumina or Pt, Rh/alumina catalyst as supported on a honeycomb support.
As to the technique as disclosed in JP-A-241467/1995, long-term stable effective oxidation removal of the CO from the exhaust gases, as discharged from the dilute combustion gas engines, needs to set the quantity of the Pt, as supported on the honeycomb support along with the alumina, in the range of 1.2 to 2.5 g/liter and, in the case where the quantity of the supported Pt is decreased to not larger than 1 g/liter, the long-term stable effective oxidation removal of the CO from the exhaust gases is impossible. In addition, the technique as disclosed in JP-A-241468/1995 reports that if the Pt/alumina catalyst as supported on the honeycomb support is replaced with the Pt, Pd/alumina or Pt, Rh/alumina catalyst as supported on the honeycomb support, then the quantity of the supported Pt can be selected without the problems of the technique of JP-A-241467/1995, specifically, the restriction such that the quantity of the supported Pt must be increased into a certain specific range for the purpose of the long-term stable effective oxidation removal of the CO from the exhaust gases. However, it is necessary therefor to support the Pd and Rh, which are expensive noble metals similar to the Pt, in almost the same quantity as of the Pt.
As to a CO removal catalyst on which a noble metal is supported, if the quantity of the noble metal as supported is increased, the catalytic function is expected to be enhanced, but the cost of the noble metal as a material rises correspondingly to the increase of the supporting quantity, so the economical performance is inferior. Furthermore, in the case where the exhaust gases contain SOx, the conversion of SO2→SO3 is so high that there occur problems of such as corrosion of piping by SO3, and further that the catalytic performance is greatly deteriorated by SO3.